1. Field of the Invention
The present invention relates to a video signal scrambling system and, more particularly, to a video signal scrambling system used upon broadcasting of a subscription TV, and preventing illegal reception more effectively.
2. Description of the Related Art
In a pay TV broadcasting system, in order to prevent illegal reception by a person who does not subscribe for the pay TV, video signals are scrambled and transmitted. On the receiver side, the scrambled signals are decoded to reproduce a normal video signal.
In an example of a conventional scrambling technique, a transmitting unit transmits a video intermediate frequency signal obtained by level-compressing a sync signal component; the transmitting unit also transmits scrambling data obtained by AM-modulating an audio intermediate frequency signal to inform the compression position to the receiving unit. On the receiving side, this audio intermediate frequency signal is AM-detected to reproduce the scrambling data, thus determining the compression position of the sync signal. A sync signal component corresponding to the compression position is expanded.
In such a scrambling technique, since a pulse with a rectangular wave is superposed on the audio intermediate frequency signal when the scrambling data is transmitted, a trigger pulse may be mixed in the audio signal itself to undesirably generate a buzz.
A system which solves the above problem is disclosed in Japanese Patent Application No. 62-223348 (U.S. Pat. No. 4,901,361 to Taneka et al, issued Feb. 13, 1990) assigned to the assignee of the present invention.
In this system, two sine waves, i.e., fH (horizontal frequency) and nfH, are combined to produce scrambling data. The scrambling data is superposed on an audio intermediate frequency signal, and the obtained data is transmitted. In the receiving unit, the audio intermediate frequency signal is AM-detected to separate the signal into two sine waves, i.e., fH and nfH. The contents of the scrambling data are checked in accordance with the combination of the two sine waves to determine the sync compression position, and an expansion pulse is obtained based on the determined position.
In such a scrambling technique, fH and nfH sine waves must be generated and detected (in the receiving unit) with high precision. In addition, since the accurate phase relationship between the fH and nfH waves is required, high precision with respect to a slice level upon detection is also required. Therefore, an expensive circuit is required as the entire system.